As compared with coated type magnetic recording media whose magnetic layer mainly comprising a ferromagnetic powder and a binder resin, ferromagnetic metal thin film type magnetic recording media comprising a non-magnetic support having formed thereon a thin film of a ferromagnetic metal by vacuum film formation, such as vacuum evaporation, sputtering, and ion plating, exhibit excellent electromagnetic conversion characteristics because of no use of a binder resin in the magnetic layer and have been attracting attention as media for high-density magnetic recording. Some of the ferromagnetic metal thin film type media have so far been put to practical use.
In particular, vacuum evaporation is advantageous over other film formation methods in that a metal film can be formed in a dry process at a relatively high rate and with relative ease. With respect to vacuum evaporated metal thin film type. magnetic recording media, various proposals for improvement have been made, e.g., in U.S. Pat. Nos. 3,342,632 and 3,342,633.
An outstanding problem waiting for a solution in putting ferromagnetic metal thin film type magnetic recording media to practical use is running properties and durability. That is, a magnetic recording medium is demanded not only to be durable but to run stably and smoothly while sliding at a high speed with a magnetic head for recording, reproduction and erasion.
However, the ferromagnetic metal thin film type recording tapes involve difficulty in maintaining running durability as compared with coated type magnetic tapes. This is because the coated type magnetic recording tapes contain a lubricant in the magnetic layer and therefore maintain surface lubrication for an extended period of time whereas metal thin film type magnetic recording media have a lubricant layer on their surface and, once the lubricant falls off, the magnetic layer is ready to be damaged.
In order to satisfy running durability of the ferromagnetic metal thin film type media, it has been proposed to form a protective or lubricant layer comprising a thermoplastic resin, a thermosetting resin, a fatty acid, a fatty acid metal salt, a fatty acid ester, or an alkyl phosphate on the surface of a magnetic-layer as disclosed in JP-A-60-69824 and JP-A-60-85427 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Use of a fluorine-containing compound as a protective material has also been proposed in various modifications. For example, JP-A-61-107528 discloses use of a compound containing a branched perfluoroalkenyl group for improvement in durability; U.S. Pat. No. 3,778,308 discloses use of a perfluoroalkyl polyether type compound; and JP-B-60-10368 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses as a protective compound a perfluoroalkyl polyether with one or both of its terminals modified with a polar group such as a carboxyl group. Further, Sugiyama, et al. report their study on the use of a multi-chain perfluoroalkyl polyether amide which is obtained by dehydrating condensation of a compound having a plurality of amino groups and a carboxylic acid having a perfluoroalkyl polyether chain as a lubricant for a ferromagnetic metal thin film type magnetic recording disc (see Abstract B.multidot.28 of the 34th National Conference of Japan Lubrication Society, p 425).
JP-A-64-33713, JP-A-1-112516, JP-A-3-102614, JP-A-1-112528, and JP-A-62-192029 disclose that compounds having a polar group such as isocyanate compounds and oxo acids, improve running durability.
JP-A-3-207021 discloses that durability at a still mode is improved by vacuum depositing a polar group-containing lubricant on a metallic film and then applying thereon a liquid lubricant having weaker polarity than that of the polar group-containing lubricant.
JP-A-63-237216 discloses improvement of durability chiefly of a plated magnetic layer of a magnetic disc against a magnetic head by coating a perfluoropolyether on a protective film comprising a hydrophilic group-containing fluorine compound.
JP-A-61-113126 discloses a metal thin film type magnetic recording medium which is freed of tape squeaks on running or clogging of a magnetic head by providing a layer containing both a polar group-containing perfluoropolyether and a perfluoropolyether containing no polar group on a magnetic layer.
JP-A-4-76816 discloses to improve running properties and durability of a metal thin film type magnetic recording medium by using a combination of a lubricant having a viscosity of not more than 30 cSt and a lubricant having a viscosity of not less than 30 cSt.
Summarizing the above-mentioned prior art, in a magnetic recording medium comprising a weakly polar perfluoropolyether alone as a lubricant is apt to be easily released during running, while a magnetic recording medium comprising a polar group-containing perfluoropolyether alone as a lubricant is short of fluid lubrication and therefore insufficient for still durability or frequently causes clogging attributable to a non-adsorbed lubricant. Accordingly, many of conventional techniques aim at elimination of these disadvantages by using a polar group-containing perfluoropolyether in combination with a perfluoropolyether with little polarity.
From this viewpoint, among the so far proposed lubrication techniques, the techniques disclosed in JP-A-63-237216 and JP-A-3-207021 are deemed to furnish a fairly satisfactory system for improving running durability.
Nevertheless, the technique of JP-A-63-237216 is confined to application on a protective layer. Besides, the expected effects are not manifested in some cases unless the coating conditions are properly selected. The lubrication technique of JP-A-3-207021 involves vacuum evaporation needing a complicated step, and widening of a non-magnetic support meets difficulty in controlling the evaporation deposit thickness.
On the other hand, where the lubricants are coated on a ferromagnetic metal thin film, the running durability of the resulting magnetic layer is influenced by the adsorption (concentration of adsorbate) and degree of adhesion of the lubricant, which factors are also associated with the surface condition of the metal thin film. In this connection, a number of surface treatments for improving surface properties of a ferromagnetic metal thin film have been proposed. Such surface treatments include (1) application of a steam stream heated at 100.degree. C. or higher onto the surface of a ferromagnetic metal thin film to form a protective layer for improving corrosion resistance (JP-A-57-123533), (2) heat treatment of a magnetic metal thin film in contact with water to form an anticorrosive protective layer (JP-A-57-138053), (3) exposure of a ferromagnetic metal thin film to a glow discharge in a water- and nitrogen-containing atmosphere to form an anticorrosive and durable oxide layer (JP-A-59-119542), (4) exposure of a ferromagnetic metal thin film to a glow discharge in a water- and argon-containing atmosphere to form an anticorrosive and durable oxide layer (JP-A-59-119543), (5) heat treatment of a magnetic metal layer in roll form in an oxygen- and water-containing atmosphere to form an anticorrosive and durable uniform oxide layer (JP-A-63-9021), (6) firing the surface of a ferromagnetic metal thin film mainly comprising cobalt at a temperature of 200.degree. to 290.degree. C. to form a cobalt oxide layer which has excellent abrasion resistance and on which a lubricant (exemplified by silicone oil and polymers) can be fixed (JP-B-49-29445), (7) treatment of a magnetic metal thin film layer in water or an aqueous solution containing a surfactant and an organic solvent to improve weather resistance (JP-A-59-2231), (8) treatment of a ferromagnetic metal thin film with an alcohol containing 10 to 80% by weight of water followed by formation of a layer comprising an organic compound to improve durability and weather resistance (JP-A-63-229620), and (9) heat treatment of a ferromagnetic metal thin film in the presence of oxygen and water to improve abrasion resistance (recommended treating conditions are 40.degree. C., 40% RH to 90.degree. C., 15% RH, and a recommended lubricant is a perfluoropolyether) (JP-A-62-287426).
Of these known surface treatments, the techniques (1) to (5), (7) and (9) consist of formation of a protective layer, such as an oxide layer, by combining a heat treatment or a glow discharge treatment and a contact treatment with at least water, and the technique (6) consists of formation of a cobalt oxide layer by heating. The technique (8) employs a surface treatment with a mixed solution of water and an organic solvent. The former techniques (i.e., (1) to (6), (7) and (9)) are attended by a problem that the surface treatment sometimes results in a reduction in strength of the magnetic layer, making it brittle and deteriorating durability. In the latter technique (i.e., (8)), the organic solvent used is adsorbed on the surface of the magnetic layer to interfere with adsorption of a lubricant onto the magnetic layer, sometimes resulting in clogging.
That is, none of the known techniques proposed to date has afforded an effective solution to the above-described problems.